Render using way too much memory no matter how much I lower settings
Every step you listed except for clamping probably helped out.
Turning clamping to 0 actually unclamps them completely.  You need some value in there, I think indirect is clamped at 10 by default and unclamping it increases render times pretty significantly.  It probably wiped out all the gains made by the other methods.
As for the memory usage, check your materials for duplicate image textures.  If you're using high-resolution textures and you think you could get away with a lower resolution, that will take less memory.
You can go this menu -

and lower your viewport settings and maybe cut down on the geometry that blender has to keep track of outside of the rendering process.
Consider adaptive subdivision if you have any highly subdivided meshes.